Recently, relatively smaller platforms have been developed for deep sea operations where marginal production does not merit the use of a full scale tension-leg platform (TLP). These marginal platforms use tension-leg mooring, like conventional tension-leg platforms, but comprise smaller floatation structures. Tension-leg mooring typically comprises rigid, single-piece tendons for anchoring the structure to the sea floor, like that disclosed in Monopod TLP Improves Deepwater Economics, PETROLEUM ENGINEER INTERNATIONAL (January 1993), incorporated herein by reference.
It is desirable to use the TLP as a production platform, however, the weight and cost of the TLP platform increase significantly with increasing water depth and payload. Monohull vessels provide greater capacities, but they can hardly support the great weight of the risers which transport minerals from wells on the sea floor when disconnect from the operating site is required. Therefore, it is desirable to use a small riser wellhead TLP platform to suspend the risers and support the wellheads, and a disconnectable tender vessel to receive, process and export the produced minerals. The well streams will be chocked and manifolded on the TLP riser platform.
Close mooring of the TLP riser platform and the production tender vessel allows for light weight flexible hoses to be used to transport the minerals. However, as the two vessels are brought into close proximity so that minerals can be transported to and from the TLP to the tender vessel, environmental loads induce excessive displacements on the two vessels and large loads on the transfer system.
Most mooring systems are heavy for large water depths and rough environments so that they require larger production and tender vessels. In marginal deep sea production, larger vessels are not economical. Previously, mooring systems have been provided for single vessels. For example, U.S. Pat. No. 5,045,415, issued to Marshall on Oct. 8, 1991, incorporated herein by reference, discloses a mooring bridle. The reference also suggests that multiple vessels may be moored within the same mooring bridle. But, in order to moor vessels relative to each other so that minerals may be transferred between by light weight, flexible systems, these prior systems require a significant number of mooring lines which increases the overall weight of the system.
Therefore, there is a need for a mooring system of two vessels which stabilizes the two vessels relative to each other without excessive loading the vessels.